This invention relates to electrical power regulation, and particularly to a regulated power supply that drives an RF generator feeding into a load of varying impedances.
Typical applications for such a power supply include the regulation of power to cathode sputtering systems for assuring that the power applied to a deposition target will remain constant during changing conditions that very not only the load but the impedance characteristics that produce appreciable quantities of power reflected back from the load to the generator.
One example of a typical application of a regulated power supply for use in cathode sputtering systems is disclosed in the U.S. Pat. No. 4,287,466 to Quick where the regulator was responsive to both the rectified RF voltage applied across the load (forward power) and to the reflected power sensed in the transmission line so that the forward power was adjusted to a level which caused only a fixed or limited amount of reflected power to be maintained, thereby causing heat dissipation to remain at a safe level and to provide a continuing reduction in forward power as losses increased because of deteriorating failure conditions.
Another example of a regulated power supply for constant output power to be dissipated in varying loads, such as encountered in cathode sputtering systems, is disclosed in the U.S. Pat. No. 4,276,591 to Quick where the circuitry measured both the rectified DC load voltage and current and generated a corresponding voltage signal which was combined into a product multiplier to obtain a product voltage proportional to the DC output power. This product voltage was compared with an externally supplied DC control signal representing the desired power output. The comparator output signal controlled a pass element that varied the saturation current in saturable reactors in the AC power input lines to the rectifier circuitry. The power supply included a limiter circuit responsive to either the voltage signals corresponding to the measured voltage, load current, or product voltage which was calibrated to a desired threshold level. When the power, voltage and/or current exceeded a desired threshold level, a limiter took control of the pass element bypassing the comparator to thus maintain the output of the power supply at a predetermined safe level.
This latter circuitry had the advantage over other types of regulated power supplies in that it did not rely on the difference in forward and reflected power reaching a certain ratio, such as disclosed in the U.S. Pat. No. 4,122,400 to Medendorp et al., but separated and treated the forward and reflected power in a manner whereby the RF generator was protected independently of any ratio.